Conventional methods of manufacturing a reflector of a light source can be classified into a method using a whiter pigment/dye additive and a method including generating air bubbles in a film.
Korean Patent No. 611599 entitled “White Film For Surface Light Source Reflection Member” discloses a white film including air bubbles, for a reflection member of a surface light source in which a coating layer including a light stabilizer is formed at least one side of the white film. Since the white film exhibits a low degradation in brightness over a period of time despite heavy usage and can maintain a high quality image for a long time, it can be preferably used for an edge light of an LCD screen, a reflector of a surface light source of a direct type light, and a member used for a reflector.
An edge light type that uses a cold cathode ray tube as an illumination light source and the light that can be illuminated form a edge of a light guide plate is widely used as an illumination member of a liquid crystal screen (Japanese Patent Publication No. 63-62104). To illuminate the liquid crystal screen, in order to efficiently use light, a reflector is installed around the cold cathode ray tube. Also, to efficiently reflect light diffused by the light guide plate toward the liquid crystal screen, a reflector is installed under the light guide plate.
For a large screen such as an LCD television, since high screen brightness cannot be provided using the edge light method, a direct type light method is used. According to the direct type light method, the cold cathode ray tubes are arranged parallel to each other above a reflector. A flat shape or one obtained by molding a portion of the cold cathode ray tube in a semicircular groove shape is used as the reflector.
Also, since the surface light source reflection member such as the reflector or the reflector used for a surface light source of an LCD device needs a high reflection function, a film containing a white pigment with a white pigment or white dyes, a film including micro-bubbles, or a metal plate or plastic plate to which the film is added. In particular, a film including micro-bubbles is widely used because it can provide good improvement of brightness or regularity (uniformity). Japanese Patent Publication No. hei 6-322153 and Japanese Patent Publication No. hei 7-118433 each disclose a film including micro-bubbles. These publications concern technologies related to 1) mono-axial extension or biaxial extension after adding film forming resin and incompatible resin, 2) mono-axial extension or biaxial extension after adding organic/inorganic particles, 3) extrusion sheet molding using foaming gas, and 4) foaming by injecting gas in an extrusion sheet.
Recently, micro-foaming polyethylene terephthalate (MCPET) products of Furukawa are widely used as a material for reflectors for backlights of LCD devices. The MCPET products have a high light reflectance by micro-foaming polyethylene terephthalate (PET) resin of a pet bottle material to form a plurality of micro-bubbles in a sheet. The following Korean patent applications are related to the above-described MCPET products.
(1) Korean Patent Application No. 10-2003-0073384 entitled “Backlight Unit”;
(2) Korean Patent Application No. 10-2003-0082948 entitled “LCD Module”;
(3) Korean Patent Application No. 10-2003-0089231 entitled “LCD Device Backlight Assembly”;
(4) Korean Patent Application No. 10-2004-0100558 entitled “LCD Device And Manufacturing Method Thereof”;
(5) Korean Patent Application No. 10-2004-0046746 entitled “Backlight Unit”;
(6) Korean Patent Application No. 10-2004-0078310 entitled “Optical Film And Backlight Unit Having The Same”;
(7) Korean Patent Application No. 10-2005-7005358 entitled “Optical Reflector And Manufacturing Method Thereof”;
(8) Korean Patent Application No. 10-2005-0010677 entitled “Backlight Assembly And Display Device Having The Same”; and
(9) Korean Patent Application No. 10-2005-0016992 entitled “Backlight Unit And LCD Device”.
Products commercialized using the above conventional material for a reflector for a surface light source and major characteristics thereof are shown in Table 1 below.
TABLE 1ManufacturerTeijinTeijinTorayMitsuiFurukawaDupontDupontProductTDFJTDFJPET-188PP(polypropylene)-MCPETNameUX-100UX-150100Thickness100 μm150 μm188 μm100 μm940 μmDensity 1.2 1.2 1.00.27Reflectance97.698.697.29897.9ManufacturingCO-PET +CO-PET +PET + TPXPP(polypropylene) +PET + gasTechnologypigmentpigment(polymethylefilter + extensionpentene)OthersHeattreatment +polishingprocessapply
FIG. 1 illustrates the structure of each of the commercialized products of Table 1. The conventional products shown in FIG. 1 and Table 1 obtain a desired reflectance by added pigment/dyes, forming air bubbles inside, and extending pores through extension after adding an additive.
FIG. 2 illustrates the principle of diffuse reflection of a reflector. Total reflection is a total of mirror reflection and diffuse reflection. Mirror reflection of light occurs when light is directly reflected from a surface of a reflector so that a reflected wave proceeds in a particular direction. Diffuse reflection occurs when light incident on micro-bubbles, white pigments, and crystalline particles in a reflector is reflected in all directions and externally emitted. A degree of the diffuse reflection is determined by the numbers of the micro-bubbles, the white pigments, and the crystalline particles in the reflector. In an LCD, it is advantageous to have a higher diffuse reflectance.
The conventional products of Teijin Dupont, Toray, and Mitsui using the white pigment/dyes additive have reflectors which are manufactured using an additive such as CaCO3 or BaSO4 having a relatively superior reflectance. The reflectors are manufactured to be thin because of the material costs of the white pigment/dyes additive. That is, when the thickness of the reflector increases, the amount of the white pigment/dyes additive included in the reflector increases accordingly, which radically increases the overall material cost. When the reflector is manufactured to be thin, the overall size of the reflector is limited. That is, when the overall size of the reflector only is increased in a state in which the reflector is thin, the reflector is twisted or warped by heat when used for a long time or may not have a sufficient structural strength so that it cannot be applied to a large screen TV over 42 inches.
To address the above problem, the foaming product of Furukawa can be manufactured to be thick and applied to a large screen TV over 42 inches. However, in order to foam PET, a gas absorption process that is performed in a high-pressure gas tank for 48 hours is needed, thereby increasing production costs. Also, it is difficult to reduce a production time for the foaming product.